Several nucleotide-polymerizing enzymes present in calf thymus gland are under intensive investigation. These include: Terminal deoxynucleotidyl transferase, an enzyme that polymerizes any deoxynucleoside triphosphate in the presence of a suitable oligodeoxynucleotide initiator; terminal riboadenylate transferase (polyA polymerase), an enzyme that adds adenylate residues to polyribonucleotides; 3.4S replicative deoxynucleotidyl transferase, a DNA polymerase present in the cell nucleus; and 6-8S replicative deoxynucleotidyl transferase, the major DNA polymerase present only in the cytoplasm. These enzymes are purified and used in mechanistic studies. The products formed are used in further studies on mechanisms of information transfer in biological systems. Biological studies on the levels of these nucleotide-polymerizing enzyme activities in resting eukaryotic cells, differentiating cells, normal proliferating cells and malignant cells are also carried out.